I wanted to share this fix with the community. It's the solution to an issue I was experiencing today with a redirection loop error.
When I first installed WordPress on my site back in 2009 I used two upper case letters in the URL. Never had an issue until today when I decided to change to a multisite configuration and use the domain mapping plugin. Every time I tried to Network Activate the plugin I encountered a redirection loop issue and couldn't get into the backend.
After a lot of research on the web I eventually came upon the solution. The issue was being caused by the site URL. When I first installed WordPress, I'd entered the URL as theLightMatters.com rather than thelightmatters.com. Changing the URL back to all lower case fixed the issue and now everything works like a charm.
Making the change is straightforward enough but does mean editing the wp-config.php file and using phpMyAdmin to edit four tables in the database: wp_site, wp_blogs, wp_sitemeta and wp_options. Look for the URL and make the necessary changes. If you don't feel comfortable doing this, find someone who can do it on your behalf.
Hope that helps,
BTW, huge thank you for developing such a great plugin.